Telecoils, also known as t-coils, are coils of wire wrapped around a core that induce an electric current when in the presence of a changing magnetic field. Telecoils are used as an alternate or supplemental input device for hearing aids and cochlear implants. Originally, telecoils in hearing aids and cochlear implants were used to pick up electromagnetic signals naturally generated by older telephones having speakers driven by powerful electromagnets. More recently, hearing aid compatible phones may include additional magnetics to generate the magnetic fields. Moreover, telecoils may be used with hearing assistive technology, such as hearing loops, which can improve listenability in meetings, classrooms, theaters, churches, and the like. For example, a hearing loop may be a room loop, neck loop, or silhouette that receives audio signals and passes them through an induction loop to create a magnetic induction field detectable by nearby telecoils. The magnetic signals received by a telecoil are converted to electrical audio signals and provided to a user of the hearing aid or cochlear implant. In many cases, using a telecoil may provide an improved listening experience because, for example, by turning off the normal hearing aid microphone, the user receives the desired audio signal without other interfering background noise surrounding the user.
A common problem with telecoils is that, in addition to the pickup of desired magnetic signals, the telecoils are also sensitive to stray magnetic hum fields that are often present in various indoor and outdoor environments. For example, some power lines, building wiring, fluorescent lights, dimmer switches, heavy electrical equipment, and the like may generate electromagnetic interference. The electromagnetic interference may generally occur at power frequencies (e.g., 60 Hz in North America and 50 Hz in Europe and most of Asia) and their harmonics.
FIG. 1 is a graph of the frequency response of exemplary stray magnetic hum fields measured at various locations in an indoor environment. Referring to FIG. 1, it can be seen that the strength of the harmonics decreases with frequency and that the even harmonics (120, 240, 360 Hz, etc.) are virtually absent. The spectrum is dominated by the odd harmonics (60, 180, 300 Hz, etc.), resulting from symmetrical distortion of the 60 Hz fundamental.
A telecoil circuit, as typically implemented, does not respond equally to all these frequencies. FIG. 2 is a graph of the frequency response of exemplary stray magnetic hum fields picked up by a typical telecoil at the various locations in the indoor environment of FIG. 1. Referring to FIG. 2, the typical telecoil frequency response rolls the spectrum off at 6 dB/octave below 1 kHz.
Further, the sensitivity of an ear, whether impaired and corrected or not, is not uniform with frequency, especially at quieter listening levels. FIG. 3 is a graph of the frequency response of one of the exemplary stray magnetic hum fields as picked up by a typical telecoil at the location in the indoor environment of FIG. 2 and the frequency response of the one exemplary stray magnetic hum field as perceived by a typical hearing aid user based on an applied weighting curve. Referring to FIG. 3, a subjective weighting curve, drawn from the spectral weighting defined in ANSI standard C63.19 for the measurement of cellphone-generated interference, has been applied to the exemplary stray magnetic hum field (labeled as “Unweighted hum”) to generate a spectrum (labeled as “HA-weighted hum”) as perceived by a typical hearing aid user.
While the telecoil frequency response and the subjective weighting reduce the relative effect of the lower hum frequencies, 180 Hz in particular still appears dominant. Prior efforts at reducing the subjective annoyance of the hum by filtering off low frequencies below 200 or 300 Hz have not been satisfactory, however, because the higher “buzz” frequencies remain untouched. These harmonics, from 300 Hz to roughly 1 kHz are directly within the voice band. Sonion Microtronic sells amplified telecoils that include an extra roll-off below 217 Hz, which provides essentially no positive subjective effect. A previous prototyped attempt by Etymotic Research included a fourth order 320 Hz high-pass filter that effectively reduced the dominant 180 Hz component, but left untouched the audible higher voice band harmonics.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.